AVweb's John Deakin concludes his six-part powerplant management series with a discussion of the procedures he uses during descent, approach (including missed-approach), landing, and shutdown. In the process, he debunks some Old Wives' Tales about
November 12, 2000
|About the Author ...
John Deakin is a 35,000-hour pilot who worked his way up the aviation food chain
via charter, corporate, and cargo flying; spent five years in Southeast Asia
with Air America; 33 years with Japan Airlines, mostly as a 747 captain; and
now flies the Gulfstream IV for a West Coast operator.
He also flies his own
V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft
scene, flying the C-46, M-404, DC-3, F8F Bearcat, Constellation, B-29, and
others. He is also a National Designated Pilot Examiner (NDPER), able to give
type ratings and check rides on 43 different aircraft types.
let's tie the ribbons on this series by discussing the right way to manage our
turbocharged powerplant at the end of our flight when it's time to get our
airplane down from altitude and back on terra firma. (We've been airborne for
five months now, and frankly I'm ready to stretch my legs and take a pit
Some will call it heresy, but I'm not a big believer in "shock
cooling," and I'm certainly not a fan of some of the exaggerated
techniques used by some to combat it. I was amazed the first time I heard of
the technique of limiting MP reductions to not more than one inch in two
minutes. I can sympathize with owners who will go to extreme measures to
protect their very expensive engines, but I've always felt that an inch of MP
per two minutes goes well beyond extreme.
There's no data, of course. With large fleets of airplanes, highly
standardized operations and detailed record keeping, we might come up with
some evidence one way or the other on this issue, but that's simply not going
But that doesn't mean I yank power off, or move any controls in a rough or
abrupt manner, either! With a little knowledge, and decent instrumentation,
it's pretty easy to control the engine temperatures, and still get the job
done with very little extra workload.
[I am a real fanatic about reducing workload, but some will call
that laziness. I hate "busy work." One of the things I like about
lean-of-peak (LOP) operation is that it's less work, once you know how, and
become accustomed to it.]
Speaking of shock cooling, I'd like to show you a related graph of some
very recent data from my own airplane. I was at 16,500 feet, normal cruise
when a fuel tank ran dry. It caught me by complete surprise, the engine quit,
sputtered halfway to life on a little bit of extra fuel, then quit cold again.
I had a bottle of water sitting on top of the fuel selector, so I had to move
that before selecting the other tank. I just happened to have my computer
running, and logging data at the time. I snipped out a couple of minutes of
that data around the event, and graphed them for you.
Click for a high-resolution version.
On this chart, the left side scale shows only EGT and TIT. All other data
is referenced to the RIGHT side of the chart. All data is read directly,
except fuel flow, which I have multiplied by 10 to get it on the chart.
To me, the most remarkable thing about this chart is how little change
there was! At the 18-second mark, the fuel runs out, as evidenced by the red
fuel flow line dropping sharply. Within six seconds, I got the fuel back on,
and within another six seconds, the engine was running normally again. I put
the boost pump on low for a moment, until the engine caught, but it wasn't
The TIT and the EGTs dropped only about 50°F or 60°F, the CDT (Compressor
Discharge Temperature) dropped about 20°F, probably from the turbo slowing
down a bit. The IAT (Induction Air Temperature) dropped only slightly,
following the CDT.
But, look at the CHTs! They hardly moved at all, dropping slowly about
10°F, and then recovering very slowly.
The most severe case is the need for a rapid descent from high altitude,
and if we run through some of the techniques for that case, then we have
covered the bases. Once you understand how to handle this "crash
dive," the methods for doing the gentler descents should be obvious.
The first rule in avoiding "shock cooling" is to not let the CHTs
get too hot to begin with! It is physically impossible to "shock
cool" anything that is not already "too hot." If there is any
merit to the conventional wisdom about "shock cooling," I suggest
that is probably mostly based on experiences in which the engine, at cruise,
at the point of the initial power reduction, was already operating well into
the 410°F to 440°F range, which is much "too hot." If you keep the
CHTs cooler, say down around 380°F, then it becomes very much harder to
accept the idea that you are going to "shock cool" the CHT by even
large power reductions. If you understand and follow the practices outlined
below, I believe "shock cooling" will become a much less common
subject for hangar talk.
Many believe it is safe to fly at
airspeeds in the yellow arc if conditions are smooth. That's not what
the FAA intends the regulations say the yellow arc is for "for
inadvertent, momentary overspeed only." (2004 correction by the author: That language does not currently appear in the FAA regulations. It does appear in several operating manuals, but those are not regulatory. My opinion of trespassing into the yellow remains unchanged, but I do apologize for misquoting the FAA regs.)
Talk to anyone who has ever crossed the
wake of another airplane. Some of those encounters have produced 10g on
recording g-meters, and some may have been responsible for a few of the
more mysterious in-flight breakups. Also, how many times have you seen
turbulence go from "none," to "ouch, that hurts"
almost instantly? This is very common over mountainous terrain. What do
you do when that happens, you're "too fast," and you want to
slow down? Why, you pull the nose up, right? Wrong, maybe, that just
adds to the loading on the wings, even if only momentarily. My
suggestion is to stay out of the yellow in the first place, and if you
need to slow down, get the power back. Be VERY careful raising the nose
in a sudden turbulence situation.
Turbulence speeds are not chosen because
they feel good, they are mathematically calculated, and it's fairly
straightforward. You take the square root of the design load factor
(3.8g in most of our small airplanes, only 2.5g in transports), and
multiply the result by the stalling speed. The square root of 3.8
(roughly 4) is (roughly) 2, so your turbulence speed will be (roughly)
twice the clean stalling speed in almost any general aviation aircraft
in the "Normal" category. 60 knots at the 1g stall (straight
and level) means that you will stall at 120 knots if you pull 4.0g ...
for any reason. In effect, if you are at or below the published
turbulence speed for your airplane, the wings will stall before anything
on the airplane breaks.
What is REALLY scary is if you run some
of the higher numbers. Suppose you are at 160 knots in the same
airplane, and hit the wake from a 757 that passed a few minutes ago? It
will now take 7.0g to stall the wings! Will your forty-year-old airplane
take 7.0g without damage? Suppose you're at 180 knots? Your wings will
not stall until you hit 9.0g!
Pay attention to that yellow arc, and in
general, stay out of it.
Let us assume we are cruising merrily along at the usual 17,500 feet, 210
knots true, with the hottest CHT around 380°F, and the TIT around 1550°F.
Everyone aboard is experienced at clearing his or her ears during a rapid
descent. Of course, we're LOP. We know there's turbulence at lower altitudes,
or there's a thick layer of icing conditions, or perhaps you know that Center
is going to keep you high, then give you a "crowbar."
"Crowbar," you ask? That's where you open the cockpit window,
throw a crowbar out, and then try to beat it to the ground. Just consider one
of those times you've looked longingly at the ads for speed brakes. Now, let
me show you why you don't need them!
As a point of reference, no matter how you handle the engine during
descent, approach, landing, and taxi to the hangar, you'll rarely find your
CHT very far from 290ºF or 300ºF, as you get ready to shut down. That means
a gradual cooling from about 380ºF to 300ºF should satisfy even the most
particular pilot, right? That's only 80 degrees!
If you're really worried about this, why not "pre-cool" the
engine? There are several easy ways to do this:
- Drop the nose and begin a very gentle descent a little early with no
power change. You'll see the CHT drop with the increased cooling airflow.
- Lean the engine even more, in small stages, remembering, "Leaner is
cooler." Also note that a bit leaner at this point will produce less
power, which will help you get down.
- Reduce 100 RPM at a time, to 2100 or even less (unless your tach has a
yellow or red arc that prohibits this, of course). This will further cool
the engine, and reduce power.
- Open the cowl flaps a little.
Any of those will start the "pre-cooling" process, and they may
be used together, in any combination to get the job done, to reduce your
engine temperatures at any rate you want, to any value. You can easily drop
the CHTs to 290°F or so, and keep them there all the way to the hangar.
When you are happy with the results, and cannot reduce power any more with
leaning and/or lower RPM, it's time for yet another "Big Pull," but
this time it's the throttle. Just firmly pull it right back to 18 or 20 inches
of MP (from 31 inches), within a few seconds.
Let us pause here a few minutes, to give the "inch in two
minutes" folks a chance to recover, for most of them have fainted dead
away at this thought!
On my airplane, this particular "big pull" will hardly change the
EGT, TIT, or CHT at all! How can this be? Well, either by serendipity or good
design, if I am running WOTLOPSOP ("Wide Open Throttle, Lean of Peak,
Standard Operating Procedure"), and simply pull the MP back to 18 inches
or so, the linkages are just about perfect to leave me at the far lower MP
setting, but just on the RICH side of peak EGT, without ever touching the
mixture control. I'm usually unable to resist fiddling with the mixture a bit
(Hmmm, what DID I just say about workload?) I enrich a little and see the EGT
fall (indicating ROP operation). I'll turn the knob back towards lean a bit,
find peak EGT (or TIT), then I enrich it slightly, usually ending up right
where I was, right after the big throttle pull.
This leaves us with about 18 inches of MP, 2100 RPM, and stable engine
temperatures. You will need the usual further reductions in throttle during
the descent, as increasing atmospheric pressure causes the MP to rise, and as
the IAS increases towards (or into) the yellow arc.
Rapid descent is the one time you WANT the absolutely hottest temperatures
you can get, to go along with the very low power you need to get down. By
controlling the POWER with the THROTTLE, and the TEMPERATURE with the MIXTURE,
you can very neatly accomplish a descent of up to 2,500 FPM in a Bonanza!
Many general aviation pilots customarily just push the mixture to full rich
when beginning descent. I shudder at the thought! They are not only getting
the cooling from the low power and excess airspeed, but they are throwing all
that extra fuel in, adding to the cooling! Furthermore, if they have been
cruising at high altitude in a turbocharged aircraft, the fuel in the tanks
has cold soaked, and it may now be -10 or -20°C, itself. And you are spraying
gobs of that fuel out of a fuel injector, directly onto the cylinder head
structure in the area around the intake valve. Would you go out to a very hot
engine and deliberately squirt a stream of cold water on one of the very
hottest parts of the engine? That is precisely what you do when you decide to
run move the mixture to full rich under those conditions.
Now THAT is "shock cooling," if you want to believe in the
concept at all!
These tricks will easily give us about 1,000 fpm descent. Want more? Slow
down, extend the gear, then descend at the maximum gear speed. (This works
best if you have a fairly high gear speed, and may not be effective at all
with a low gear speed.) One word of caution here, if you expect ice that you
will not shed before landing, you might want to reconsider gear extension. It
may get you down through the icing layer more quickly, but it may also leave
you with a lot of ice on the gear, adding drag, and the gear may foul it if
you try to retract it. This one is best used if you expect to keep the gear
down all the way to the landing, or you're SURE there's no ice in the descent,
or there is a known warm layer to melt the ice off. Like so many things in
aviation, it's a judgment call.
The approach is nothing more than a continuation of the descent, as far as
engine management goes. By the time you arrive at the approach fix, or the
beginning of your pattern entry, your engine (and your turbo) should be cooled
to just about the coldest it's going to get. My CHTs run a consistent 290ºF
at the end of this cooling process (during descent). You can mess this up by
boring holes in the sky with your gear down and high power, which runs the
temps back up again, or you can use just enough power to sneak around in the
clean configuration until you need the gear.
The subject of minimum CHT comes up fairly often. Few manuals show a real
limitation. In the big radials, you will often find a figure somewhere around
100°C (212°F) as minimum for takeoff. The metallurgy isn't much different,
and the principles are the same, so that would seem to be pretty strong
evidence. If an engine can go straight to full power (sometimes 60 inches of
MP, 2800 RPM) with a CHT that low, then there can't be much worry about damage
to the engine! Some of the big radials have suggestions to maintain CHTs
within a fairly narrow range in cruise, but that is for efficiency, not to
"save the engine."
According to George Braly of GAMI:
Many POHs give the normal "operating
range" for CHT for TCM engines as being between 240°F and 460°F. The
operating specifications in the TCM manual do not even list a
"minimum" temperature for CHTs, although in one place, TCM
suggests not going below 300°F for more than five minutes. It is hard to
reconcile these various different recommendations. It is even harder to
reconcile this discrepancy when one considers that a number of big bore TCM
engine installations, in a variety of well baffled aircraft, will cruise
with several cylinder heads at temperatures as low as 280°F to 290°F, even
at high power settings. Some of these engines have lots of time on them with
absolutely no engine problems.
One set of cylinders we have some data on
were used for customer demonstrations in a turbo engine installation. The
aircraft was repeatedly climbed at full power to 18,000 feet, operated at
very high power cruise for 30 minutes or so, then put through a low power,
high rate of descent, "slam dunk" back to near sea level. The CHTs
were often down to the 250°F to 260°F range during the later portion of
those demonstration descents. The descents were otherwise conducted in a
manner very similar to that described in this article. Those cylinders were
pulled for detailed dimensional inspection after 800 hours of that kind of
severe operation. The cylinder barrels and pistons all remained within new
dimensional limits, including the critical choke areas of the barrels. The
cylinders were completely free of cracks of any kind. Due to the operating
environment, these cylinders probably experienced several times the
aggregate "cold shock" abuse than would any normal private owner's
engine during even two or more TBO runs! But note, those cylinders were not
typically operated over 380°F during cruise, and the low power descents
were managed in the manner described, so as to not dump cold fuel into the
cylinders and so as to maintain the EGTs and TIT at maximum values during
the descents, all of which had the overall effect of minimizing the rate of
change in the cylinder head temperatures.
PLEASE don't run your prop up to high RPM in the pattern, or you'll
contribute to the ill will at the next town meeting to close the airport.
If you haven't touched the prop and mixture controls prior to the landing,
there's no need to mess with them now. If you have reset them, take a moment
to re-lean the mixture as suggested for taxi-out.
You need to fix the power management for this procedure firmly in your
mind, practice it thoroughly, and stick to it every time. The traditional
method is "Mixture (rich), Prop (full forward), and Throttle (full
forward)," in that order.
The most conservative method is to set your mixture to full rich before you
enter the pattern (after you've cooled things down as above).
You may wish to modify this slightly with some engines at some airports, as
they may be set up a bit too rich for full power at sea level, and the
non-supercharged engines will definitely need to be leaned for best power at
It is more important to be mentally prepared for this perfectly normal
maneuver than to have all the controls in the cockpit set for it. It is
perfectly safe to make the approach AND landing with the prop set to a low
RPM, and the mixture set to LOP, PROVIDED you train yourself to take the above
actions in a reasonably expeditious (but not rushed) manner. You MUST practice
this, until it becomes second nature. For many pilots, just jamming the
throttle in is about all they can be expected to remember, and for them, the
classic approach is best.
I would guess that when you see (and hear!) someone running their engine in
the parking spot after landing "to cool the turbo," you are almost
always seeing someone who is getting it hotter than it was when he arrived at
the spot. If you don't make the high-drag approaches, and you lean brutally
after landing, that turbo will be just about as cold as it will ever get,
short of a shutdown. On the other hand, if you've made a long, high-drag
approach with high power, or you're one of those dreadful types who taxi with
cruise power while dragging the brakes to keep the speed down, or you have a
long, sharply uphill taxi to the ramp, you may need the cool down. A tip that
might do your engine more good is to hop right out after shutdown, and pop the
cowling open (or even just the oil access door) if you can. That will let a
LOT of heat out of the accessory section, avoiding "cooking" the
components, hoses, and seals there. But even that little tip is probably
completely unnecessary if there is more than 3-5 knots of wind blowing in the
general direction of either the front or the back (cowl flaps open, please) of
George Braly again:
We know of one operator of a turbo twin
Cessna. He owned the aircraft for 15 years before the issue of "turbo
cool down" ever showed up in the aviation literature. Through three
full TBO runs, with almost no premature engine or turbo problems, this
operator would routinely land at his uncontrolled airport, turn off at
mid-field, taxi about 100 yards, and immediately shut down both engines. The
aircraft was then promptly pushed into a T-hangar, where it could not
benefit from any natural wind for a further cool down. These would have been
ideal conditions to promote problems with turbochargers not being properly
allowed to cool down, if there was any truth to this OWT. This operator, in
more than 8000 engine hours, never experienced any problems with the issue
of "coking" a bearing on a turbocharger.
Finally, here's a graph of the type of descent I've just described.
Click for a high-resolution version.
The chart begins at 11:49:21 PDT, on 10/02/2000, at 10,500 feet. Note that
TIT and EGT is read against the left side numbers, while all other parameters
are read against the right side. For the first five minutes, I pre-cool the
CHTs by beginning a gentle descent, and reducing RPM and fuel flow on the lean
side. That CHT drop too fast for you? Fine, you have complete control over how
fast you drop the CHTs, take all the time you want.
At about 11:55:00, I simply pulled the throttle back, and dropped the MP
from 31.0" to 18.0". In order to demonstrate where the mixture ended
up to a passenger, I fiddled with it a bit, and added just a bit of fuel. In
fact, it would probably have been better if I'd just left it alone (as I
usually do) because that extra tweak actually caused the CHTs and EGTs to
RISE! Uhh, what WERE you saying about "shock cooling"? We have just
pulled off 12 full inches of MP in one swell foop, and all engine temps go UP!
The reason is, of course, we have gone from very LOP (and relatively cool CHTs)
at a very high power setting, to just ROP at a very low setting. The linkages
in my engine are "just right" for this purpose, yours may vary, but
at worst, you'll need to fiddle with the mixture once, to get it just ROP, or
wherever you want it.
For the next 10 minutes on the chart, we descend at about 1,000 fpm, at
about 140 knots, gear up. If I'd wanted more descent, all I had to do was run
the IAS up to the bottom of the yellow arc, or put the gear down, or even pull
off more MP. For a couple minutes in the pattern, the CHTs dropped very gently
and then they rise a bit while taxiing in. At shutdown, they begin a long,
slow cooling process.
Well, we've gone from startup to shutdown, with a "flight time"
of five months! I hope I've not led you too far astray, and above all, I hope
I've made you think.
I think I'll take a break from engines, for a while!
Be careful up there!